<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        // 9. 将div#demo.demo转化成<div id="demo" class="demo"></div>
        function change(str) {
            let re = /([a-z]+)\#([a-z]+)\.([a-z]+)/g
            let newstr = str.replace(re,function(match,$1,$2,$3){
                return "<" + $1 +" id=" + $2 +" class="+$3+"></"+$1+">"
            })
            console.log(newstr)
        }
        change("div#demo.demo")
    </script>
</body>
</html>